1. Field of the Invention
The present invention relates generally to metal working, and more specifically, to an apparatus and method for pecision forming of sheet metal parts without transfer drilling. The invention also relates to three-dimensional structures made using the apparatus and method, wherein attachment holes are punched before bending in order to eliminate transfer drilling during assembly.
2. Description of the Related Art
The manufacturer of large, geometrically complex structures, such as radar antennas, normally involves extensive machining, e.g., transfer drilling, in order to assemble components of a particular structure. In prior methods of forming reflector ribs of a radar antenna, a series of flat contour-punched plates with pre-located attachment holes are assembled with pre-drilled, machined extrusion corner clips which are riveted to each end of the rib, and then riveted to the main vertical ribs. Contour-punching is generally known in the art, and refers to the act of cutting a sheet to a desired shape by making a series of punched cuts.
The assembly of these parts is then performed in a holding fixture which supports the outboard ribs in their pre-determined position. The remaining major and minor ribs are then sequentially fastened. At the final assembly, the contoured reflector assembly is positioned in its required attitude and fastened to an independent support structure by transfer drilling and bolted through a series of reflector support angles.
The support structure is made of a series of sheet metal box beams, each of which is formed by assembling pre-punched flat panels and sheet metal channels which are bent up to conventional sheet metal tolerances. Internal bulkheads used in the assembly are conventional two-part structures which allow for take-up of dimensional variation in the beam dimensions. All sheet metal parts are contour-punched and bent from a reference edge of the flat, pre-bent sheet. In order to join the sheet metal to cast frame members, the parts are transfer drilled and rivets are installed in the drilled holes.
Transfer drilling has been required in the past because sheet metal bending techniques have not been capable of providing sufficiently close tolerances such that pre-punched holes would align between juxtaposed structural parts after bending.
The inventors herein developed a new method in which each plate is at first contour-punched and then hole-punched to include attachment holes and bending tool holes. The plate ends were then precisely bent from the bending tool holes to form integral mounting flanges. After bending, the attachment holes were disposed in the mounting flanges. By using the bending tool holes to position the metal sheet on the bending machine, the attachment holes, which were pre-punched, were capable of being located within precise dimensional tolerances previously achievable only by machining. Since machining is both costly and time consuming, the pre-punching of attachment holes provided fabrication of three-dimensional assemblies with a minor number of parts and a minimum amount of transfer drilling at assembly. The attachment holes of one part precisely matched those in an adjacent part or complementary part in forming a structural component of an assembly. The use of integral end flanges eliminated the need for separate corner clips since, at each joint, rivets were inserted through the common holes, thereby reducing the total number of attachments.
The method developed by the inventors also permitted the integration of contoured reflective parts into a main support structure of the reflector assembly. Selected main, vertical and horizontal contoured ribs were joined together by precision bent sheet metal channels riveted between a pair of ribs. The matching attachment hole patterns in the ribs and channels were punched prior to forming. Also, hole patterns for internal bulkheads were pre-punched to match corresponding holes in both the contoured ribs and the channels. Cast frames and bulkheads in the assembly were all drilled by a machine controlled by a numeric control unit so as to match the geometry and hole patterns in the adjoining sheet metal parts. Thus, even though the attachment holes are bent into a different plane, tolerances of about 0.005 inch are achievable over the entire length of the bent portion of the sheet metal. In the past, bending sheet metal up to about ten feet long was performed without pre-punching attachment holes since, the achievement of close tolerances for attachment holes along the entire length would have been extremely difficult.
The method described above uses a standard bending machine in which an upper die descends towards a lower die as tonnage is applied through opposite side rams. A lower die used in the standard bending machine which facilitates the method of forming sheet metal in which sheets are first punched and then bent is considered part of the present invention.
A problem common to the die of the present invention, which permits close tolerance bending, and other dies, is that sometimes when the bending machine rams cause the upper die to descend, unequal pressure from the rams may cause the upper die to move unevenly downward. Also, imperfections in either the upper die or lower die, or both, may cause the upper die to contact the hole-punched sheet unevenly, thus causing the sheet to move slightly relative to the die. This problem is detrimental to maintaining close tolerances and is particularly acute for metal sheets made of aluminum due to the bending characteristics of aluminum.